Separation system for polymeric blast media

ABSTRACT

A process and apparatus is described for recovering and cleaning for reuse a lightweight blast media typically a polymeric material that is employed as a gas and entrained abrasive for cleaning workpiece surfaces of dirt, corrosion scale, paint or the like. The process involves reentraining the contaminated abrasive material in a gas stream from the worksite. The entrained material was directed in to a separator which separates the abrasives and heavier contaminants from the gas stream by impinging onto a target. Heavier material cascades from the target into the separator collection zone while the lightweight contaminants are swept from the separation zone by means of a vacuum system. The contaminated abrasive media is further cleaned by separating blast media of acceptable size from oversize and finer contaminants, by means of triple screening. The blast media, subjected to a series of separators, achieves a finished product having a 3-5% contaminant content. The process and apparatus of the invention are particularly suitable for recovering polymeric blast media.

This application is a continuation of U.S. patent application Ser. No.794,124, filed Nov. 1, 1985, now abandoned.

TECHNICAL FIELD

The invention relates to processes and apparatus for blast cleaning ofworkpiece surfaces to remove dirt, corrosion scale and the like. Moreparticularly of interest is recovering and cleaning blast media forreuse.

BACKGROUND ART

In recent years, lightweight polymeric blast media have replacedagricultural products, such as walnut shells, apricot pits and the like,for use in blast cleaning of sensitive workpieces. The polymeric blastmaterial is more consistent in its cutting qualities, as well as inertto degradation by environmental factors.

Polymeric blast media are relatively expensive and, in general, must berecovered for reuse. The cleanliness of the abrasive material, however,needs to be precisely controlled where the workpieces to be cleanedinclude surface tolerances that are critical and the integrity of thesubstrate cannot be compromised.

Where the blast cleaning operation is directed to relatively smallarticles and workpieces that can be blasted within a confiningapparatus, such recovery for reuse is not difficult. However, where theworkpiece to be cleaned is of large size, such as an airplane, the blastmedia must first be recovered from the work site before it can becleaned and recycled.

After collection of the blast media from the work site, contaminationsuch as paint flakes, corrosion particles and dirt must be separatedfrom the polymeric media. The contaminants include large and finematerial about the same weight as the blast media. Also present arefines and dusts that are lighter in weight than the blast media.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide a system and process forrecovering and cleaning for reuse a lightweight blast media, typicallypolymeric, that is employed as a gas-entrained abrasive for cleaningworkpiece surfaces of dirt, corrosion scale and the like.

The process and apparatus of recovering the lightweight media from a jobsite entails, first of all, re-entraining the contaminated abrasivematerial in a gas stream. The entrained contaminated abrasive media aredirected into a separator which separates the abrasives and the heavierportion of contaminants from the gas stream by impinging the gas streamonto a target. The target causes a reversal of gas flow and absorbssufficient momentum of the heavier contaminats and the abrasivematerial, wherein this relatively heavier material cascades downwardsfrom the target while the lightweight fines material remains entrainedin the gas stream.

The lightweight fines contaminants and dust remaining in suspension inthe gas stream are swept from the separator in a manner which impartssuffcient swirl and turbulance to the gas stream to sweep thelightweight contaminants from the interior of the separator and from thecascading contaminated media. The falling contaminated abrasive materialis collected at the base of the separator. It may then be re-entrainedin a gas stream for a series of air washings similar to the first.

In a preferred system, a separator discharges the heavier fraction ofthe abrasive media and contaminants onto a screen system for furthercleaning. The screens reject particles larger than the polymericabrasive media, allowing the heavier fraction of the fines to passthrough the screen system to waste, and accept that fraction ofparticles which are equivalent to the average blast media size range.The accepts fraction may be subjected to a succeeding air wash cleaningbefore dropping into a hopper for reuse.

The system for recovering and cleaning the polymeric blast media forreuse includes a first vacuum system for re-entraining the spent,contaminated abrasive into a gas stream from a blast work site. Theentrained media are directed into a closed-top separator tank designedto separate the lightweight fines portion of the contaminants from theabrasive media material and heavier contaminants. The tank includes abottom discharge from which accepted material is discharged for furthercleaning or to a storage hopper for reuse.

The closed top of the separator includes a first closed-end cylinder ofsmaller diameter than the tank, open to the tank interior at its bottom,wherein the closed end is a target for receiving the gas stream in whichthe contaminated abrasive blast media material is entrained. The targetcylinder causes a reversal of the impacting stream, wherein theabrasives and heavier contaminants cascade to the bottom discharge ofthe tank while the lightweight fines, contaminants and dust remain insuspension in the gas stream. The top of the separator tank includes asecond closed-end cylinder of smaller diameter than the tank, saidcylinder open to the tank interior at its bottom and having a sidewalltangential discharge for discharging the fines-contaminated gas streamexiting the separator interior. A second vacuum system evacuates theseparator tank interior through the second closed-end cylinder in amanner that causes the airstream to swirl through the cascading heavierabrasives and contaminants falling to the bottom of the tank, sweepingthe lightweight fines from the media and separator interior.

The system includes a series of screens for removing contaminants fromthe blast media by size. A first screen removes oversized contaminantsgreater than 30 mesh. Smaller fines than 80 mesh pass to waste also.Those materials retained on an 80-mesh screen are substantially thecleaned, lightweight polymeric blast media. The media may then either bedirected for reuse or subjected to a polishing separator similar to thefirst air wash separator. The resulting 95-97% pure polymeric blastmedia are then suitable for reuse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing the recovery and cleaning systemof the invention.

FIG. 2 is a schematic drawing showing a plan view of the separator tank,including the first and second closed-end cylinders and the air currentsgenerated interior to the separator by evacuation of fines contaminants.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIGS. 1 and 2, the system of the invention is shown inschematic detail. A workpiece which, in the illustrated case, is anairplane 10, is subjected to a blast cleaning operation to remove paintfrom fuselage surfaces. A nozzle 11 directs a relatively soft polymericblast-cleaning media 12 of controlled aggressiveness onto the aircraftsurfaces. The spent blast media and contaminating debris, such as dirtand paint flakes, fall to the ground about the aircraft. A suction pump13 enables a workman using a suction line 14 to re-entrain the spent,contaminated blast media. Discharge from pump 13 is directed to a firstair wash separator tank 15. Discharge line 16 projects the entrainedabrasive particulate and contaminants into a closed-end cylinder 17,which is open at its base into the interior of the separator tank 15.The closed-end cylinder 17 causes the gas stream in which thecontaminanted abrasive media are entrained to reverse direction,resulting in the blast media and heavier contaminants 18 cascading intoa hopper collection zone 19 of the separator tank 15. The lightweightportion of the contamination, such as dust and light paint flakes,remains suspended in the separation zone 20 of the tank in the freespace above the collected material 18.

The lightweight contaminants are evacuated from the tank through asecond closed-end cylinder 21, also shown in the plan view in FIG. 2.The second closed-end cylinder 21 includes a sidewall tangentialdischarge 22 which imparts a swirling motion to the gases in theseparator zone 20 to help keep the fines material from settling outanywhere in the separator and to sweep through the cascading acceptedmaterial to further remove fines from the tank and cascading media. Asecond vacuum pump 23 evacuates the separator tank 15 through thetangential discharge 22.

The contaminated blast media 18 accumulate in a pile at the base 19 ofthe separator tank 15. A discharge gate 24 meters the material into anentrainment system 25. The re-entrained contaminated blast media isdirected through a magnetic separator 26, which removes ferrouscontaminates, and then is directed into a second separator tank 27,which is substantially of the same design and function as the firstseparator tank 15. The contaminated blast media from which more fineshave been removed now settle in the base of the separator tank 27 forfeed through a feed gate 28 into a screening system.

A first screen 29, typically of 30 mesh, rejects oversized contaminantsfrom the contaminated blast media. A second screen 30, typically of 80mesh, retains blast media and allows heavy fines to pass therethrough toa disposal bin 31. The substantially clean blast media drop into are-entrainment system 32, which transports the media into a polishingair wash cleaner 33, which, in design and function, is similar to thefirst separator system 15.

The cleaned blast media are fed through a feed gate 34 into a feedhopper 35. The hopper 35 supplies the blast nozzle 11. At this reusepoint, the blast media retains only 3-5% contaminants.

In operation, the blast media are vacuumed from the job site andinjected at about 2800 feet per minute into the separator unit 15. Thedistance from the exit of the injector pipe 16 to the top of the dome ofthe target cylinder 17 is selected such that, in combination with thegas stream velocity and character of the blast material, the blastmaterial drops from the entrainment gas in a substantially uniformlydistributed curtain cascading downward to the collection zone 19.

A preferred polymeric blast media is an amino-base thermoset resin,Polyplus™, manufactured by U.S. Plastic and Chemical Corporation ofPutnam, Conn. The blast media material has a bulk density of 58-60pounds per cubic foot and is discharged at 2800 feet per minute into thefirst closed cylinder, which is 20 inches in diameter. The separatortank of such a system is 60 inches in diameter.

In this preferred system, the second vacuum pump 23 exerts a 13-15 inchHg vacuum on the separator tanks generating less than 2800 feet perminute exiting fines-contaminated gas streams.

With the Polyplus™ blast media, the screen system for removing heavyoversize and fines is designed to recover a 30-80 mesh particle sizerange.

In a preferred operation, the entrained, contaminated blast media isinjected into the first closed-end cylinder with a velocity such thatabrasive media material does not impact the top of the closed-endcylinder. The object is to have the blast media lose momentum under theinfluence of gravity and gas resistance therein to cascade downward in auniform curtain, avoiding agglomeration, so that the falling materialcan be subjected to the swirling, exiting gas stream, sweeping the dustand light contaminants from the cascading material. The exiting gasstream is limited in velocity so that none of the polymeric blast mediaexit with the fines. There must be sufficient velocity so that none ofthe dust settles out or accumulates inside the separator.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

We claim:
 1. A system for recovering and cleaning for reuse acontaminated lightweight material employed as a gas-entrained abrasivefor cleaning workpiece surfaces of contaminants such as dirt, corrosion,scale and the like, where the abrasive has a specific gravity less thanthe heavier contaminants and greater than the lightweight contaminantscombined therewith as a result of the cleaning operation, comprising:afirst vacuum system for re-entraining spent, contaminated abrasive intoa transporting gas stream from a work site, said first vacuum systemdischarging the gas stream in a generally vertical direction; aclosed-top separator tank including a bottom portion for collection anddischarge of said abrasive and an upper portion, said upper portionincluding: a first closed-end cylinder including a target surface towardwhich said gas stream containing the contaminated abrasive is directedupon discharge from said first vacuum system, said first closed-endcylinder having a shape to cause a disbursal and general reversal indirection of travel of the abrasive and contaminants, said firstclosed-end cylinder including a substantially unrestricted bottomdischarge opening through which the abrasive and contaminants falldownwardly at a substantially reduced speed of travel toward said bottomportion of said separator tank with the lightweight abrasive andcontaminants therein remaining substantially suspended while the bulk ofthe abrasive and heavier contaminants travel to said bottom portion ofsaid separator tank for removal; and a second closed-end cylinder ofsmaller diameter than said separator tank, said second cylinder havingan opening open to said tank upper portion interior at its bottom at aposition lateral of said first closed-end cylinder opening communicatingwith a substantially dead air space of said separator tank and having asidewall tangential discharge for discharging said gas streamcontaminated with lightweight contaminants from the upper portion ofsaid separator tank interior, said second cylinder opening being out ofdirect path of the falling bulk of the abrasive and heavier contaminantsto capture the suspended lightweight contaminants and lightweightabrasive particles; and a second vacuum system for evacuating saidseparator tank through said second cylinder tangential exit, wherein agentle swirling motion is imparted to the lightweight abrasive particlesand contaminants throughout said separator upper portion by operation ofthe second cylinder tangential discharge, such that said exiting gassweeps the lightweight contaminants from the cascading heavier abrasivesfalling to the bottom of the tank, and said abrasives, cleaned of saidlightweight contaminants are discharged from said tank bottom discharge.2. The recovery and cleaning system of claim 1 wherein said contaminatedgas stream is injected substantially vertically into said impact targetat a velocity wherein all material as heavy as clean abrasive materialcascades downward from said impact target into the bottom portion ofsaid separator.
 3. The recovery and cleaning system of claim 1, furtherincluding a dust collector for receiving the fines-contaminated gasstream exiting said second cylinder tangential discharge and removingsaid fines from said gas stream.
 4. The recovery and cleaning system ofclaim 1, including a screen for receiving the heavier abrasive materialdischarging from said tank bottom discharge, said screen removingcontaminating debris larger than said abrasive material.
 5. The recoveryand cleaning system of claim 4 wherein said screen removes materialgreater in size than 30 mesh from said contaminated abrasive material.6. The recovery and cleaning system of claim 5 wherein said screenretains abrasive material of greater than 80 mesh for reuse in abrasivecleaning and permits finer debris and heavy fines to pass therethroughfor disposal.
 7. The recovery and cleaning system of claim 4 wherein amagnetic separator for removing ferrous contaminating debris is includedprior to said screen.
 8. The recovery and cleaning system of claim 1wherein said lightweight contaminant separator system includes aplurality of such separators, in series, wherein the bottom discharge ofeach separator, except the last separator, is connected to a vacuumsystem for re-entraining said partially cleaned material in a gas streamwhich is then directed into a first target cylinder of the subsequenttank, said last tank having a bottom discharge discharging cleanabrasive material into a hopper for reuse in abrasive cleaning.